You have to keep looking around in all of the rooms everywhere on that floor. You will find a room with a person who will teach you the new spell. Take that spell and use it to open the door with spell marked on that door. Also, I have found new spells in treasure chests. I picked up a really powerful Ice spell(uses almost the entire magic bar).
And, Yes! this game is very playable without knowing japanese. There are 2 gameplay modes. One mode is specifically a 'Dungeon Hack' mode where it is mostly fighting. The first mode, which is exploration mode even is playable without knowing a translation. But, this game is sooo good, that you actually want to know what is going on! But, it doesn't seem entirely necessary. Well atleast as far as I have gotten. Which, still seems relatively early. I think this game may be HUGE.